This invention relates to an apparatus for detecting properties of sheet material, such as bank notes or papers of value, by reflected light according to the preamble of the main claim.
EP-A 0 537 531, DE-A 38 15 375 and DE-A 195 32 877 disclose apparatuses wherein sheet material is transported along a transport direction past an illuminating device and a detecting device. The illuminating device illuminates the sheet material along a line perpendicular to the transport direction. Light reflected by the sheet material along the line is detected by a detecting device. The latter either contains a CCD array or a photodiode array, the design and arrangement of which determine the local resolution of the reflected light along the line. The line is generally selected to be wider than the width of the sheet material perpendicular to the transport direction so that the total surface of the sheet material can be detected during transport.
The abovementioned apparatuses permit good detection specifically of diffusely reflected light. However, if the sheet material contains specularly reflecting components, such as metallic-luster security threads or printing inks or components incorporated in the sheet material or so-called OVDs, the latter cannot be distinguished from diffusely reflecting areas of the sheet material in all cases, in particular if their reflectivity is not very great.
U.S. Pat. No. 5,299,268 shows an apparatus like the above-described apparatuses wherein the sheet material is additionally illuminated along a second line perpendicular to the transport direction. The formation of suitable intensity ratios of the light intensities detected along the two lines permits detection of specularly reflecting components of the sheet material.
This apparatus is disadvantageous in that, firstly, relatively high equipment expense and, secondly, a relatively complicated evaluation method are necessary for detecting specularly reflecting components of the sheet material.
In addition, none of the apparatuses permits magnetic material properties of the sheet material components to be derived.
Briefly, in accordance with the invention, an apparatus is disclosed for detecting properties of sheet material along a line with specularly reflected light which is of simple construction and preferably also permits magnetic properties of the sheet material components to be derived.
This problem is solved according to the invention by the features stated in the main claim. Advantageous developments are the subject of the subclaims.
The basic idea of the invention is to provide an imaging device which converts light emitted by an illuminating device into a substantially parallel light pencil which illuminates a line in a line plane parallel to the sheet material and falls in a first illumination plane at a fixed angle of incidence, the first illumination plane forming with the line plane a first illumination angle and intersecting the line plane along the line. The imaging device is disposed as close to the line as possible. Light reflected along the line in a first detection plane at a fixed angle of reflection is imaged onto a detecting device, the detection plane forming with the sheet material a first detection angle and intersecting the line plane along the line. The angle of incidence is selected equals to the angle of reflection, and the first illumination angle equals to the first detection angle. For deriving magnetic properties of the sheet material components, the illuminating device preferably emits polarized light, and light specularly reflected along the line is analyzed by means of a polarizer before the detecting device.
An advantage of the inventive solution is that the apparatus is of simple construction and the intensities detected by the detecting device substantially contain only the fractions of specularly reflected light, so that one can do without an elaborate evaluation method for the detected intensities. The use of polarized light for illumination and suitable analysis of specularly reflected light by means of a polarizer additionally permits magnetic properties of the sheet material components to be derived.
For detection of specularly reflecting components of the sheet material and their magnetic properties, the sheet material is guided in the line plane so that light emitted by the illuminating device hits the sheet material directly and is reflected by the specularly reflecting components of the sheet material.
For detection of magnetic properties of non-specularly reflecting components, a reflector preferably having a high magnetic Kerr effect is provided in the line plane. Sheet material is guided past said reflector for detection so that the permanently magnetized magnetic components of the sheet material influence the Kerr effect of the reflector through magnetization thereof, which results in a change of light intensity reflected on the reflector.
In the following, various embodiments of the invention and the functional principles thereof will be described with reference to the figures, in which: